An optical network has long enjoyed the sub-60 ms self-healing ring architecture. As the network grows, the ring topology is no longer suitable of its cumbersome provisioning and complex ring inter-working in a large network. The optical mesh network helps solve some of these issues. However, it suffers from a historically slow fault recovery time. The ring network is able to achieve sub-60 ms protection time because the fault detection and protection switching are performed locally where the fault occurred. In a mesh network, the fault recovery procedure is executed at the source and destination for end-to-end path protection. As a result, the fault notification time has contributed to slow recovery time for a mesh network.
Most optical transport networks today are based on electronic switching equipment which takes light as an input and converts the light into electronic data. It then processes the electronic data and converts them back to light. In order to carry the data, different types of framing protocol have been developed, such as, SONET (synchronous optical network), SDH (synchronous digital hierarchy), and OTN (optical transport network). The framing protocol uses a relatively small amount of bandwidth for its overhead data to carry framing information, error checking and monitoring, fault notification, and etc. For example, SONET AIS/RDI (alarm indication signal/remote defect indication) protocols may be used to notify terminating nodes of an optical circuit of the fault condition in the network.
An optical network is a collection of optical network devices interconnected by links made up of optical fibers. Thus, an optical network is a network in which the physical layer technology is fiber-optic cable. Cable trunks are interconnected with optical cross-connects (OXCs), and signals are added and dropped at optical add/drop multiplexers (OADMs). The optical network devices that allow traffic to enter and/or exit the optical network are referred to as access nodes; in contrast, any optical network devices that do not are referred to as pass-thru nodes (an optical network need not have any pass-thru nodes). Each optical link interconnects two optical network devices and typically includes an optical fiber to carry traffic in both directions. There may be multiple optical links between two optical network devices.
A given fiber can carry multiple communication channels simultaneously through a technique called wavelength division multiplexing (WDM), which is a form of frequency division multiplexing (FDM). When implementing WDM, each of multiple carrier wavelengths (or, equivalently, frequencies or colors) is used to provide a communication channel. Thus, a single fiber looks like multiple virtual fibers, with each virtual fiber carrying a different data stream. Each of these data streams may be a single data stream, or may be a time division multiplex (TDM) data stream. Each of the wavelengths used for these channels is often referred to as a lambda.
A lightpath is a one-way path in an optical network for which the lambda does not change. For a given lightpath, the optical nodes at which its path begins and ends are respectively called the source node and the destination node; the nodes (if any) on the lightpath in-between the source and destination nodes are called intermediate nodes. An optical circuit is a bi-directional, end-to-end (between the access nodes providing the ingress to and egress from the optical network for the traffic carried by that optical circuit) path through the optical network. Each of the two directions of an optical circuit is made up of one or more lightpaths. Specifically, when a given direction of the end-to-end path of an optical circuit will use a single wavelength, then a single end-to-end lightpath is provisioned for that direction (the source and destination nodes of that lightpath are access nodes of the optical network and are the same as the end nodes of the optical circuit). However, in the case where a single wavelength for a given direction will not be used, wavelength conversion is necessary and two or more lightpaths are provisioned for that direction of the end-to-end path of the optical circuit. Thus, a lightpath comprises a lambda and a path (the series of optical nodes (and, of course, the interconnecting links) through which traffic is carried with that lambda).
FIGS. 1A and 1B are block diagrams illustrating an optical circuit of a typical SONET/SDH based optical network. In the SONET/SDH world, AIS/RDI signals are generated by the first node that detects a failure of loss of signal (LOS) (e.g., a loss of an electrical signal) in order to suppress the alarms. Both AIS and RDI may be used as triggers to initiate a protection switch action. Note that because the SONET based network assumes that the signal is fully regenerated at each node, only one node would ever detect a LOS on its ingress. On the egress of that node, it would still send a framed SONET signal that contained null data with alarm information in its overhead. Downstream nodes would thus not detect a LOS but would see AIS in the overhead. The downstream node would associate defects related to that signal to the fault reported by an upstream node.
Referring to FIG. 1A, where there is a unidirectional path failure, the intermediate node C is the first node to detect such a failure. Since each of the nodes in the SONET/SDH based network regenerates signals at its respective egress. The downstream of the path (e.g., nodes D and E) still receive optical signals. Typically, in response to the detection, intermediate node C sends AIS signals to both downstream nodes to notify the fault conditions. The terminating node (e.g., node E) may return an RDI signal to its upstream nodes (e.g., nodes A-D) of the optical circuit.
In a case of bi-directional path failures, as shown in FIG. 1B, both nodes B and C send AIS signals to their respective downstream adjacent nodes (e.g., nodes D and A) for the notification purposes. The downstream nodes that receive such notification signals may rebroadcast the notification messages (e.g., an AIS signal) to its respective adjacent downstream nodes.
As a result, each of the intermediate nodes may be required to receive such notification messages, convert the optical notification messages into electrical signals, and regenerate another notification message to its adjacent nodes.
Photonic switching equipment (e.g., equipment that does not typically perform optical to electrical conversion of switching, with exception of adding and dropping traffic) used in all-optical networks, although not widely deployed yet, it is typically based on the GMPLS architecture. The GMPLS architecture also uses signaling protocols, such as RSVP-TE, to perform hop-by-hop data path establishment, removing, and fault notification. When a fault on a data path is detected, a fault notification message is sent hop-by-hop to the source and destination nodes. Such a notification relies on the transmission speed of the signaling channel which is typically 10/100 Mbps.
Such notification messages (e.g., AIS/RDI or RSVP-TE) may be queued during the transmission (e.g., particularly, during the signal conversions between the electrical domain and the optical domain). As the optical network grows, particularly, in a mesh optical network, such notification messages are getting larger and larger which put a heavy burden on the network traffic. As a result, the fault notification may be delayed significantly. Furthermore, in a typical network element, an alternative route may not be established until a routing database is updated, which may take relatively longer time.